An interior light, a rear light, a flasher light or the like is recently controlled to produce gradually changing brightness of light to provide fanciness as electric equipment of a vehicle or produce light of fixed brightness irrespective of changes in a power supply voltage of a battery. The brightness of a light source such as a light bulb and a light-emitting diode is controlled by driving a switching element such as a MOSFET by pulse-width modulation (PWM) control. Since the number of electric loads such as lights to be driven increases recently, radio noises, which are generated at time of driving the electric loads, increase correspondingly.
As technology for reducing radio noises generated when a PWM control is performed, as disclosed in the following patent documents 1 and 2, it is proposed to shape a waveform of a PWM signal into a trapezoid form so that a current supplied to a load changes slowly or gradually thereby suppressing noises. According to this technology, a voltage applied to a gate of a switching element such as a MOSFET changes at low levels during a period, in which a current waveform changes at a uniform rate. Since the switching element continues to have a high on-resistance during such a period, the switching element generates heat. The amount of heat thus generated increases, as the amount of a drive current increases or the number of electric loads to be driven increases. As a result, it becomes difficult to promote heat radiation. Since reduction in radio noise and suppression of heat generation are in a relation of trade-off, the radio noise reduction and the heat generation suppression need be appropriately adjusted.
Patent document 1: JP 3685108 (US 2002/0171455 A1)
Patent document 2: JP 2009-16697A
Patent document 3: JP 2007-13916A (US 2006/0267665 A1)
In patent document 1, for example, trade-off between radio noise reduction and heat generation suppression is considered. As a result, a very complicated circuit configuration is needed. In patent document 3, a waveform of a PWM signal is modified to a pseudo-sinusoidal waveform, which is more effective to reduce noise generation and suppress heat generation.
Specifically, in patent document 3, the pseudo-sinusoidal waveform is generated by sequentially switching connections of a plurality of constant current sources while monitoring a source voltage of a FET. According to this feedback control, circuit configuration need be complicated and the circuit becomes more susceptible to noises generated by other devices. If the signal waveform is smoothed by simply using a CR filter, rise of the signal waveform is smoothed but fall of the same is not smoothed.